Sodium Site Exchange and Migration in a Polar Stuffed-Cristobalite Framework Structure

Alberto J Fernández-Carrión; Aydar Rakhmatullin; Li Yang; Michael J Pitcher; Dominique Massiot; Florence Porcher; Mathieu Allix; Xiaojun Kuang

doi:10.1021/acs.inorgchem.1c00319

Sodium Site Exchange and Migration in a Polar Stuffed-Cristobalite Framework Structure

Inorg Chem. 2021 Apr 5;60(7):4322-4331. doi: 10.1021/acs.inorgchem.1c00319. Epub 2021 Mar 13.

Authors

Alberto J Fernández-Carrión¹, Aydar Rakhmatullin², Li Yang¹, Michael J Pitcher², Dominique Massiot², Florence Porcher³, Mathieu Allix², Xiaojun Kuang^{1

4}

Affiliations

¹ MOE Key Laboratory of New Processing Technology for Nonferrous Metals and Materials, Guangxi Key Laboratory of Optical and Electronic Materials and Devices, College of Materials Science and Engineering, Guilin University of Technology, Guilin 541004, P. R. China.
² CNRS, CEMHTI, UPR 3079, Université Orléans, Orléans F-45071, France.
³ Laboratoire Léon Brillouin, CEA Saclay, Gif Sur Yvette 91191, France.
⁴ College of Chemistry and Bioengineering, Guilin University of Technology, Guilin 541004, P. R. China.

PMID: 33719424
DOI: 10.1021/acs.inorgchem.1c00319

Abstract

The study of ionic dynamics in solids is essential to understanding and developing modern energy technologies. Here we study the ionic dynamics of orthorhombic Na₂MgSiO₄, an interesting case of a polar stuffed-cristobalite-type structure that contains two inequivalent Na sites within the channels of the magnesium silicate tetrahedral framework. Its preparation by a solid-state reaction method favors the presence of ∼2% of Na vacancies, converting it into a pure Na ionic conductor with an optimized ionic conductivity of ∼10^-5 S cm^-1 at 200 °C. The macroscopic migration has been characterized through impedance spectroscopy and molecular dynamics simulation, which proves the pure Na ionic character of the compound through hopping between Na1 and Na2 sites, forming three-dimensional migration zigzag-shaped paths. High-resolution solid-state ²³Na magic-angle-spinning (MAS) NMR spectroscopy is employed to characterize the local structure and microscopic dynamics of Na-ion transport in Na₂MgSiO₄. Remarkably, variable-temperature ²³Na MAS NMR and two-dimensional exchange spectroscopy evidence for the first time a Na site exchange phenomenon at room temperature, which further triggers Na ionic conduction at elevated temperatures.